In magnetic resonance imaging, the patient must be positioned within a large magnet which provides a strong, uniform magnetic field. While the patient is positioned in the magnetic field, radiofrequency excitation signals are applied so as to elicit magnetic resonance signals. Magnetic field gradients are applied so as to affect the magnetic resonance process and thereby spatially encode the magnetic resonance signals. An image of the patient can be reconstructed from the resulting magnetic resonance signals. Because magnetic resonance imaging provides unique imaging capabilities and freedom from risks associated with other imaging modalities, it is a valuable tool for physicians.
The requirement that the patient be positioned within the magnet, however, poses unique challenges in magnetic resonance imaging. For optimum imaging, the feature of the patient's body to be imaged must be aligned with that portion of the patient-receiving space within the magnet where the magnetic field has optimum properties, commonly referred to as the imaging volume. Many magnetic resonance imaging instruments are solenoidal instruments in which the magnet is a large cylindrical structure having a horizontal central bore and coils surrounding the central bore, so as to provide an imaging volume at a particular axial location along the central bore. These instruments typically are provided with a slidable bed which can be moved into and out of the central bore in a motion like that of a common desk drawer. In this case, the instrument can be built with a laser or other marker disposed at a fixed axial distance from the axial location of the imaging volume. The technician can slide the bed until the feature to be imaged is aligned with the marker. When the feature is aligned with the marker, the feature is located at the known axial distance from the axial center of the imaging volume. Thus, after aligning the feature with the marker, the technician need only slide the bed through this known axial distance. The technician may do this manually, or by entering a command into a computer associated with the apparatus to actuate a drive mechanism. Alternatively, the apparatus can be arranged to move the bed through the known axial distance in response to a button push or other input from the technician indicating that the feature has been aligned with the marker.
Instruments of this type, however, suffer from numerous drawbacks. They provide an intensely claustrophobic experience for the patient. They are unable to accommodate extremely obese patients or patients with bulky casts or other appliances affixed to them. Moreover, they can provide images of the patient only while the patient is disposed with the long axis of his or her body horizontal, i.e., in a recumbent or prone position.
Certain apparatus disclosed in the aforementioned patents and applications, substantially overcomes these drawbacks. Such apparatus provides a magnet with a pair of pole structures such as ferromagnetic poles, superconducting coils, permanent magnets, or resistive coils disposed along a horizontal axis referred to herein as the polar axis or magnetic field axis. A patient-receiving gap is defined between the pole structures. The patient support, which may include an elongated platform, is mounted for compound movement to a variety of imaging positions. Typically, the apparatus includes a carriage which is mounted on guides such as rails for movement along a horizontal axis, referred to herein as the carriage axis, transverse to the magnetic field axis. A support structure is mounted to the carriage for pivoting movement relative to the carriage about a pivot axis. The pivot axis typically is horizontal and parallel to the field axis. The patient support is also mounted for sliding movement along the support structure so that the patient support can move along a support axis transverse to the pivot axis. Typically, the patient support includes an elongated platform extending in directions parallel to the support axis. The patient support may also include a footrest projecting from the platform at one end or a seat projecting from the platform. Drive mechanisms are provided for moving the carriage along the carriage axis, pivoting the support structure and patient support about the pivot axis and sliding the patient support along the support axis.
Systems of this type provide extraordinary versatility for the imaging process. The patient may be imaged in a substantially upright position, as, for example, while standing on the footrest and leaning against the platform; in a recumbent position, lying on the platform with the platform generally horizontal; or in any intermediate position, as, for example, a Trendelenberg or reverse-Trendelenberg position, with the platform disposed at an oblique angle to the horizontal. Moreover, systems of this type provide extraordinary ease of use. The patient support may be disposed in a load position, with the platform extending generally vertically, and the patient may be positioned on the support while the support is in this load position, as, for example, by simply sitting down on the seat or standing on the footrest and leaning against the patient support. After the patient is positioned on the support, the technician actuates the apparatus to tilt the support frame and hence the patient support to an appropriate angle, move the carriage and slide the support along the support axis so as to bring the patient support to a position where the patient is disposed at the desired orientation relative to gravity, and with the feature to be imaged disposed within the imaging volume.
However, prior to the present invention, this process has been performed by trial and error, with the technician adjusting the position of the patient support in the various degrees of freedom by entering appropriate commands into the computer which controls the drive mechanisms. This process can be time-consuming. Moreover, the technician may not accurately position the feature of interest. This, in turn, requires repositioning and restarting the imaging process.